Small dark spots appear on the leaves
at first. This is the mycelium growing. The leaf
often turns yellow and falls off weakening the rose; they may spread to the leaf stalks and the stems.
This fungus spreads by means of asexual spores (conidia) produced by the growing spots throughout the summer. These are carried in water splashes and develop into new spots especially in warm, damp weather. The infection overwinters as a mycelium on fallen leaves, in the leaf axils and as small stem infections. In spring sexual spores are produced and these transfer to the new leaves.
Often Tar Spot on sycamore is thought to be Blackspot, but there is no threat as they are different species which are specific to the two plants.
Remove any fallen leaves which will carry the infection through winter to the next growing season. Cut out any stems which show signs of infection before bud break in spring. Some varieties of roses are more resistant.
Chemical control must start in early the year as the leaf buds start to swell and become vulnerable to and spores being splashed up from debris and stem lesions. Bordeaux Mixture gives good control, but leaves a whitish deposit. Mancozeb is another agent to use and spraying should be repeated every two weeks until October. Most commercial treatments contain a wetting agent to allow the fungicide to cling to the leaves.
There is a mixture which is not approved by our masters in Brussels. If you happen to spill four level teaspoonsful of Baking Soda, a few drops of washing-up liquid and a desert spoonful of soluble fertilizer into half a jar of water which become shaken together and topple over into a gallon of water in a sprayer, then as you dispose of them accidents do happen so the roses may become covered with the spray!! It's not really carelessness if this happens every two weeks.
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